| 摘要 |
PURPOSE:To simplify the adjustment of an optical system and to prevent generation of an error due to temperature change or the like by detecting defocusing depending on a reflected luminous quantity from a focus measuring face at focusing on an information recording face and a reflected luminous quantity from a normal focus measuring face. CONSTITUTION:When a focus 9a of a laser beam of one wavelength in the laser beam radiated from a 2-wavelength hybrid semiconductor laser 1 resides on an information recording face, a reflected luminous quantity of a laser beam with other different wavelength focusing a focal point 9b on a focus measuring face 12 formed on an optical disk 8 is detected by a photodetector 14, held by signal processing circuits 17, 18, and the reflected luminous quantity thus held and the reflected luminous quantity from a focus measuring face 12 detected by the photodetector 14 at an optional time are compared to detect the quantity and direction of defocusing. Through the constitution above, since the defocus detection signal is obtained by the entire luminous quantity, the optical adjustment for focusing is not required and even when position deviation of an optical component takes place due to temperature change or vibration, no error is caused in the detection signal. |
